db/proxy: split search into chunks to avoid exceeding the output buffer

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1130

NEWS

@@ -5,6 +5,7 @@ ver 0.23 (not yet released)
 * database
   - proxy: require MPD 0.20 or later
   - proxy: require libmpdclient 2.11 or later
+  - proxy: split search into chunks to avoid exceeding the output buffer
 * output
   - pipewire: new plugin
   - snapcast: new plugin

src/db/plugins/ProxyDatabasePlugin.cxx

@@ -825,12 +825,25 @@ try {
 	const bool exact = selection.filter == nullptr ||
 		!selection.filter->HasFoldCase();
 
+	/* request only this number of songs at a time to avoid
+	   blowing the server's max_output_buffer_size limit */
+	constexpr unsigned LIMIT = 4096;
+
+	auto remaining_window = selection.window;
+
+	while (remaining_window.start < remaining_window.end) {
+		auto window = remaining_window;
+		if (window.end - window.start > LIMIT)
+			window.end = window.start + LIMIT;
+
 		if (!mpd_search_db_songs(connection, exact) ||
-	    !SendConstraints(connection, selection, selection.window) ||
+		    !SendConstraints(connection, selection, window) ||
 		    !mpd_search_commit(connection))
 			ThrowError(connection);
 
 		while (auto *song = mpd_recv_song(connection)) {
+			++window.start;
+
 			AllocatedProxySong song2(song);
 
 			if (Match(selection.filter, song2)) {
@@ -845,6 +858,15 @@ try {
 
 		if (!mpd_response_finish(connection))
 			ThrowError(connection);
+
+		if (window.start != window.end)
+			/* the other MPD has given us less than we
+			   requested - this means there's no more
+			   data */
+			break;
+
+		remaining_window.start = window.end;
+	}
 } catch (...) {
 	if (connection != nullptr)
 		mpd_search_cancel(connection);